The UK has been inhabited for longer and thus many more of its aquifers may have been tapped.
Despite being fairly arid, Australia is just massive comparatively. In combination with number 1, it may have huge aquifers under uninhabited arid regions that are close to full.
The UK has very fucked up land management, virtually all the capacity of the land to hold onto water has been destroyed in favour of dumping it into the sea as quickly as possible, the result is a country thats prone to both floods and droughts.
Two random guesses:
The UK has very fucked up land management, virtually all the capacity of the land to hold onto water has been destroyed in favour of dumping it into the sea as quickly as possible, the result is a country thats prone to both floods and droughts.